Optically variable features may be utilized for security and/or decorative purposes. For example, security features may be printed to a document. Level 1 security features may be understood as features that may be authenticated without the need for extra equipment, whereas level 2 or level 3 features may include additional or specialist equipment for verification. Level 1 security features may be provided, for example, via a visual and/or tactile effect. Level 1 security features may be provided with optically variable features, or features that change in appearance upon movement and/or viewing angle of a document. For example, holographic features may be employed, or, as another example, optically variable color shifting pigments may be utilized.
Thermal transfer printing may be used to print documents such as identification cards. Various pigments may be used as part of a thermal transfer printing process. For example, optically variable pigments may be used to provide optically variable features; however, optically variable pigments have several drawbacks. For example, optically variable pigments tend to be expensive, which renders thermal transfer ribbons utilizing optically variable pigments expensive. Further, thermal transfer ribbons may be manufactured by a gravure printing process, and optically variable pigments may have drawbacks in connection with such manufacture of thermal printing ribbons.
For example, the production of thermal transfer ribbons may utilize a pigment/polymer formulation that is applied onto a polyester film via a process such as direct gravure printing using an etched gravure cylinder. Various additive or additional polymers may be added to the formulation to assist with coating, printing, or durability performance. Generally, this production requires pigments that may be easily or conveniently suspended in a polymer solution having a coatable viscosity, and that may transfer from the cells of the gravure cylinder without blocking. However, optically variable pigments generally are of larger size, are plate-like in shape (i.e., having one dimension significantly larger than another dimension), and are of relatively high density. These attributes render optically variable pigments inappropriate or inconvenient for gravure printing. Further, difficulty in controlling the orientation of the plate-like materials renders their use difficult to control with gravure and/or thermal printing processes.